Alloy composition

ABSTRACT

An alloy composition includes from about 4 wt % to about 11 wt % of silicon based on a total wt % of the alloy composition; from about 0.1 wt % to about 0.5 wt % of chromium based on the total wt % of the alloy composition; from about 0.1 wt % to about 0.5 wt % of magnesium based on the total wt % of the alloy composition; from about 0.01 wt % to about 0.05 wt % of titanium based on the total wt % of the alloy composition; equal to or less than 0.5 wt % of iron based on the total wt % of the alloy composition; equal to or less than 0.5 wt % of manganese based on the total wt % of the alloy composition; and a balance of aluminum.

BACKGROUND

Die casting processes are commonly used to form high volume automobilecomponents. In particular, aluminum alloys are often used to form thestructural components in the die casting process because aluminum alloyshave many favorable properties, such as light weight and highdimensional stability, which allows the formation of more complex andthin wall components compared to other alloys. Traditionally, aluminumdie castings have a limitation on ductility due to air entrapment andFe-intermetallic phases. Many technologies developed for reducing theseissues, such as semisolid die casting and super-vacuum die casting, formporosity-free castings.

SUMMARY

In an example of an alloy disclosed herein, the alloy compositionincludes from about 4 wt % to about 11 wt % of silicon based on a totalwt % of the alloy composition; from about 0.1 wt % to about 0.5 wt % ofchromium based on the total wt % of the alloy composition; from about0.1 wt % to about 0.5 wt % of magnesium based on the total wt % of thealloy composition; from about 0.01 wt % to about 0.05 wt % of titaniumbased on the total wt % of the alloy composition; equal to or less than0.5 wt % of iron based on the total wt % of the alloy composition; equalto or less than 0.5 wt % of manganese based on the total wt % of thealloy composition; and a balance of aluminum.

DETAILED DESCRIPTION

Aluminum alloys often include aluminum, alloying elements (e.g., siliconand iron), and impurities. Some aluminum alloys have been developed withlow iron content. These alloys have been used for body structuralcomponents, which have superior ductility aftersolution-followed-by-precipitation hardening treatment. However, it hasbeen found that with low-iron aluminum alloys, die sticking or diesoldering occurs during the die casting process. Die sticking candestroy the casting during ejection, or even dramatically reduce dielife.

The examples disclosed herein can reduce the die sticking or diesoldering of aluminum alloy die castings through a micro-alloyingmethod. This micro-alloying method can also improve the strength of diecastings. In particular, it has been found that the combination ofparticular alloying elements in particular amounts can reduce thesolubility of iron in molten aluminum. Reducing the iron solubility, andthus the amount of dissolved iron in the molten aluminum, also reducesthe amount of iron-intermetallics that form as a result of the moltenaluminum reacting with the dissolved iron. These iron-intermetallics canstick to the die used in casting, which results in die soldering. Whendie soldering occurs, the surface finish of the resulting part (i.e.,casting, structural body) may be destroyed when ejected from the die,and the die life may be reduced as well. In the examples of the alloycomposition disclosed herein, the iron solubility is reduced, and, inturn, the die soldering is reduced.

In an aluminum-silicon alloy system (e.g., Al-10Si), the solubility ofiron is about 1.5%. Some elements, such as manganese, have been added inan attempt to reduce the solubility of iron in molten aluminum, and thusto reduce the number of iron-intermetallics that are formed. A minimumof 0.6 wt % of manganese has been found to reduce the iron solubility inAl-10Si to about 0.6%, which is about ⅔ less than the 1.5% solubility ofthe iron in the alloy without the manganese. However, it has been foundthat even this relatively small percentage of manganese can form largeintermetallics. Large intermetallics can deleteriously affect (e.g.,reduce) the ductility of a structural casting formed from this alloy(i.e., Al-10Si-0.6 Mn).

The structural casting formed from A1-10Si or Al-10Si-0.6 Mn may beexposed to a heat treatment in order to improve the ductility. However,the additional heat treatment adds an increased risk of deformation ofthe structural casting. In addition, the heat treatment increases thecost of production of the structural casting.

The examples of the alloy composition disclosed herein reduce the ironsolubility and are capable of forming structural bodies that exhibitsuitable ductility. The alloy composition disclosed herein includesaluminum, silicon, chromium, magnesium, titanium, iron, and manganese inspecific amounts. For chromium, the specific amount ranges from about0.1 wt % to about 0.5 wt %, and for manganese, the specific amount isequal to or less than 0.5 wt %. The combination of these elements inthese amounts has unexpectedly been found to further reduce the ironsolubility (when compared to the Al-10Si-0.6 Mn), to a point where mostof the iron is insoluble. For example, with the alloy composition(s)disclosed herein, the iron solubility may be as low as 0.12%, e.g., whenthe chromium amount ranges from about 0.1 wt % to about 0.2 wt %. Thisreduction in, or elimination of the iron solubility reduces oreliminates the formation of iron intermetallics, and thus also reducesthe occurrence of die soldering.

In addition, the combination of these elements in these amounts hasunexpectedly been found to form a structural casting with uncompromisedductility, even without a subsequent heat treatment. It has been foundthat the ductility may be further improved by controlling both manganeseand iron, so that each is present in the alloy composition in an amountranging from about 0.1 wt % to about 0.2 wt %.

The use of chromium in the alloy compositions disclosed herein isparticularly unexpected in foundry practice, in part because chromiumtypically leads to sludge formation. However, it has been found thatspecifically regulating the amount of chromium, in particular withrespect to the amounts of manganese and iron, can prevent sludgeformation. For example, it has been found that sludge may not form whenthe total amount of the chromium plus the manganese plus the iron isless than 0.6 wt %. In addition, tuning the amount of the chromium plusthe manganese plus the iron can advantageously reduce die sticking ordie soldering.

The chromium that is added to the alloy compositions disclosed hereinalso forms smaller intermetallics (e.g., compared to ironintermetallics) having a round-shaped morphology. Thesechromium-containing intermetallics are less detrimental to ductility,and thus may contribute to the elimination of a post-formation heattreatment (described below).

With reduced iron solubility, reduced iron intermetallic formation,improved ductility, and elimination of sludge formation, the examplealloy compositions disclosed herein may be used to form as-caststructural bodies with suitable ductility and yield strength. It is tobe understood that an as-cast structural casting, as defined herein, isthe part, component, etc. that is formed from die casting the alloycomposition, but is not exposed to an additional heat treatment. Sincethe as-cast example structural bodies disclosed herein maintain a highductility, high yield strength, or high elongation, no additional heattreatment is required. The elimination of a post-formation heattreatment reduces the cost associated with manufacturing the structuralcastings and also reduces the risk of deformation in the finalstructural casting (which can result from heat treatment).

As mentioned above, examples of the alloy composition disclosed hereinmay include silicon, chromium, magnesium, titanium, iron, manganese, anda balance of aluminum. In some examples, the alloy composition mayinclude these metals, without any other metals. Further, in someexamples, the alloy composition may exclude copper, zinc, zirconium,vanadium, or combinations thereof, or any other non-listed elements.Still further, in another example, the alloy composition may consistessentially of silicon, chromium, magnesium, titanium, iron, manganese,and a balance of aluminum. In these instances, other inevitableimpurities may be present in the alloy composition. Examples of themetals added to the alloy composition disclosed herein are discussed ingreater detail below.

The alloy composition includes silicon. Silicon may be added to thealloy composition to reduce the melting temperature of aluminum andimprove the fluidity of the molten aluminum. In an example, the siliconmay be present in the alloy composition in an amount ranging from about4 wt % to about 11 wt % based on the total wt % of the alloycomposition. In another example, the silicon may be present in an amountranging from about 4 wt % to about 7 wt %. In still a further example,the silicon may be present in an amount ranging from about 7 wt % toabout 9 wt %.

The alloy composition further includes chromium. As previously statedabove, the specific amount of chromium contributes to the reduction inthe solubility of iron in molten aluminum and also does notdeleteriously affect the ductility and/or yield strength in the finalstructural casting formed from the alloy composition(s). As such, theaddition of the specific amount of chromium may contribute to the lackof a need for an additional heat treatment of the structural castingafter the die casting process. Chromium may also improve toughness ofthe structural casting formed from the die casting process. In anexample, chromium may be present in the alloy composition in an amountranging from about 0.1 wt % to about 0.5 wt % based on the total wt % ofthe alloy composition. In another example, the chromium may be presentin an amount ranging from about 0.15 wt % to about 0.2 wt %. In stillanother example, the chromium may be present in an amount of about 0.17wt %.

The alloy composition further includes magnesium. Magnesium improves theyield strength by solid solution strengthening. Magnesium may be presentin an amount ranging from about 0.1 wt % to about 0.5 wt % based on thetotal wt % of the alloy composition. In another example, the magnesiummay be present in an amount ranging from about 0.2 wt % to about 0.5 wt%. Still further, in another example, the magnesium may be present in anamount of about 0.3 wt %.

The alloy composition also includes titanium. Titanium may be added as agrain refiner to improve the control of the grain growth of the moltenaluminum during the die casting process. In an example, the titanium maybe present in an amount ranging from about 0.01 wt % to about 0.05 wt %based on the total wt % of the alloy composition. In another example,the titanium may be present in an amount ranging from about 0.01 wt % toabout 0.02 wt %. In still another example, the titanium is present in anamount of about 0.015 wt %.

Additionally, the alloy composition also includes manganese. Themanganese may be added to reduce the die sticking or soldering. Asmentioned herein, controlling the amount of chromium, iron, andmanganese can reduce die sticking or soldering. In an example, themanganese may be present in an amount equal to or less than 0.5 wt %based on the total wt % of the alloy composition. In another example,the manganese may be present in an amount of equal to or less than 0.2wt % of the alloy composition. In yet another example, the manganese maybe present in an amount of equal to or less than 0.15 wt % of the alloycomposition. It is to be understood that the wt % of manganese isgreater than 0 wt %, and thus at least some manganese is present in thealloy composition.

The alloy composition also includes iron. Some iron may be added toimprove yield strength of the structural casting formed from the diecasting process. Iron is also included for ductility. In an example, theiron may be present in an amount equal to or less than 0.5 wt % based onthe total wt % of the alloy composition. In another example, the ironmay be present in an amount of equal to or less than 0.2 wt % of thealloy composition. It is to be understood that the wt % of iron isgreater than 0 wt %, and thus at least some iron is present in the alloycomposition.

The remainder of the alloy composition includes a balance of aluminum.In an example, the aluminum starting material used to form the aluminumin the alloy composition may be 99.9% pure aluminum with less than 0.1wt % of impurities. The impurities present in the aluminum startingmaterial may include iron, manganese, chromium, vanadium, silicon, orthe like.

In one example of the alloy composition, the alloy composition includesthe silicon present in an amount ranging from about 6 wt % to about 9 wt%, the chromium present in an amount ranging from about 0.15 wt % toabout 0.2 wt %, the magnesium present in an amount ranging from about0.2 wt % to about 0.5 wt %, the titanium present in an amount rangingfrom about 0.01 wt % to about 0.02 wt %, the iron in an amount equal toor less than 0.1 wt %, the manganese in an amount equal to or less than0.1 wt %, and a balance of aluminum. The structural casting formed fromthis example alloy has a higher ductility or elongation (without a heattreatment) when compared to other structural castings formed fromdifferent alloy compositions using the same die casting process. Thisexample alloy may attain from about 8% to about 10% elongation in theas-cast state, compared to a traditional Al—Si10-Mn (0.6) Mg alloy,which attains from about 4% to about 6% elongation in the as-cast state.

In another example of the alloy composition, the alloy compositionincludes the silicon present in an amount ranging from about 4 wt % toabout 7 wt %, the chromium present in an amount ranging from about 0.15wt % to about 0.2 wt %, the magnesium present in an amount ranging fromabout 0.2 wt % to about 0.5 wt %, the titanium present in an amountranging from about 0.01 wt % to about 0.02 wt %, the iron present in anamount equal to or less than 0.1 wt %, the manganese present in anamount equal to or less than 0.15 wt %, and a balance of aluminum. Thestructural casting formed from this example alloy has a higher averageelongation when compared to other structural castings formed from otheralloys using the same die casting process. In an example, the averageelongation of the structural casting formed from this alloy compositionmay be about 7% in the as-cast state, compared to a the traditionalAl—Si10-Mn (0.6) Mg alloy, which attains from about 4% to about 6%elongation in the as-cast state.

Additionally, the structural casting formed from this other examplealloy may have increased yield strength. In an example, the yieldstrength of the structural casting formed from this other example alloymay be about 300 MPa.

In yet another example, the alloy composition includes the siliconpresent in an amount ranging from about 9 wt % to about 11 wt %, thechromium present in an amount ranging from about 0.15 wt % to about 0.2wt %, the magnesium present in an amount ranging from about 0.1 wt % toabout 0.2 wt %, the titanium present in an amount ranging from about0.01 wt % to about 0.02 wt %, the iron present in an amount equal to orless than 0.15 wt %, the manganese present in an amount equal to lessthan 0.2 wt %, and a balance of aluminum. This example alloy forms astructural casting (body) with superior castability that does not stickto the die when ejected from the die mold. In addition, the yieldstrength may range from about 150 MPa to about 200 MPa.

In any of the example alloy compositions disclosed herein, the total wt% of the chromium plus the manganese plus the iron may be less than 0.6wt % based on the total wt % of the alloy composition. The total weightpercent of these particular alloyed elements advantageously reduces diesticking or soldering.

In an example of the method, an example of the molten alloy compositiondisclosed herein is die cast to form a structural casting. The examplesof the molten alloy composition disclosed herein reduce the diesoldering that may take place during the die casting process. Inparticular, the amount of chromium, alone or in combination with theamount of manganese, may be selected so that the iron solubility isreduced, which reduces iron-intermetallic formation and also reduces diesoldering during the die casting process. As discussed above, to reducedie soldering, the alloy composition may be formed with from about 0.1wt % to about 0.5 wt % of chromium.

Once the wt % of the chromium is selected, the alloy composition may beformed. The alloy composition may be formed by adding the alloyingelements into a pure aluminum melt. The method may also involve knowntechniques for controlling the impurity levels.

The die casting process used to make a casting from the alloycomposition may be a high-pressure die casting (HPDC) process, or alow-pressure die casting process, or a squeeze casting (or liquidforging) process. A dosing furnace with a degassing system may be usedto hold and transfer the aluminum-based melt (i.e., molten alloycomposition) to the die casting machine. The die casting processparameters may be varied, depending upon the die casting machine that isused, the size and/or shape of the casting, etc.

After the alloy composition solidifies to form the structural casting,the structural casting may be removed from the die. In an example, thecasting is ejected from the die. In some examples, the casting isremoved using ejector pins. Since soldering is reduced during diecasting, little or no scrap casting remains in the die. However, ifscrap casting remains, it may be removed from the die. Even though thealloying elements and impurities are controlled, the scrap casting maynot be suitable for recycling.

In an example, after the structural casting is removed from the die, thestructural casting is not exposed to a subsequent heat treatment and yetexhibits desirable mechanical properties (e.g., ductility, elongation,strength, etc.). The final structural casting may be an automobile part,a computer part, a communication part, or a consumer electronic part.For examples of the automobile parts, the structural casting may be analuminum-based part for the body of a vehicle, or an aluminum-basedwheel. The final structural casting may also be a part utilized in anelevator application.

Reference throughout the specification to “one example”, “anotherexample”, “an example”, and so forth, means that a particular element(e.g., feature, structure, and/or characteristic) described inconnection with the example is included in at least one exampledescribed herein, and may or may not be present in other examples. Inaddition, it is to be understood that the described elements for anyexample may be combined in any suitable manner in the various examplesunless the context clearly dictates otherwise.

It is to be understood that the ranges provided herein include thestated range and any value or sub-range within the stated range. Forexample, a range from about 7 wt % to about 9 wt % should be interpretedto include not only the explicitly recited limits of from about 7 wt %to about 9 wt %, but also to include individual values, such as 7.25 wt%, 8.25 wt %, 8.9 wt %, etc., and sub-ranges, such as from about 7.25 wt% to about 8.50 wt %, etc. Furthermore, when “about” is utilized todescribe a value, this is meant to encompass minor variations (up to +/−5%) from the stated value.

In describing and claiming the examples disclosed herein, the singularforms “a”, “an”, and “the” include plural referents unless the contextclearly dictates otherwise.

While several examples have been described in detail, it is to beunderstood that the disclosed examples may be modified. Therefore, theforegoing description is to be considered non-limiting.

What is claimed is:
 1. An alloy composition, comprising: from about 4 wt% to about 11 wt % of silicon based on a total wt % of the alloycomposition; from about 0.1 wt % to about 0.5 wt % of chromium based onthe total wt % of the alloy composition; from about 0.1 wt % to about0.5 wt % of magnesium based on the total wt % of the alloy composition;from about 0.01 wt % to about 0.05 wt % of titanium based on the totalwt % of the alloy composition; equal to or less than 0.5 wt % of ironbased on the total wt % of the alloy composition; equal to or less than0.5 wt % of manganese based on the total wt % of the alloy composition;and a balance of aluminum.
 2. The alloy composition as defined in claim1 wherein the alloy composition excludes copper, zinc, zirconium,vanadium, or combinations thereof.
 3. The alloy composition as definedin claim 1 wherein: from about 6 wt % to about 9 wt % of the silicon ispresent in the alloy composition; from about 0.15 wt % to about 0.2 wt %of the chromium is present in the alloy composition; from about 0.2 wt %to about 0.5 wt % of the magnesium is present in the alloy composition;from about 0.01 wt % to about 0.02 wt % of the titanium is present inthe alloy composition; equal to or less than 0.10 wt % of the iron ispresent in the alloy composition; and equal to or less than 0.10 wt % ofthe manganese is present in the alloy composition.
 4. The alloycomposition as defined in claim 3 wherein an as-cast structure of thealloy composition has a ductility ranging from about 10% to about 12%.5. The alloy composition as defined in claim 1 wherein: from about 4 wt% to about 7 wt % of the silicon is present in the alloy composition;from about 0.15 wt % to about 0.2 wt % of the chromium is present in thealloy composition; from about 0.2 wt % to about 0.5 wt % of themagnesium is present in the alloy composition; from about 0.01 wt % toabout 0.02 wt % of the titanium is present in the alloy composition;equal to or less than 0.10 wt % of the iron is present in the alloycomposition; and equal to or less than 0.15 wt % of the manganese ispresent in the alloy composition.
 6. The alloy composition as defined inclaim 5 wherein an as-cast structure of the alloy composition has anaverage elongation index (EI) of about 7% and a yield strength of about300 MPa.
 7. The alloy composition as defined in claim 1 wherein: fromabout 9 wt % to about 11 wt % of the silicon is present in the alloycomposition; from about 0.15 wt % to about 0.2 wt % of the chromium ispresent in the alloy composition; from about 0.1 wt % to about 0.2 wt %of the magnesium is present in the alloy composition; from about 0.01 wt% to about 0.02 wt % of the titanium is present in the alloycomposition; equal to or less than 0.15 wt % of the iron is present inthe alloy composition; and equal to or less than 0.2 wt % of themanganese is present in the alloy composition.
 8. The alloy compositionas defined in claim 1 wherein a total wt % of the chromium plus themanganese plus the iron is less than 0.6 wt % based on the total wt % ofthe alloy composition.
 9. The alloy composition as defined in claim 1wherein the aluminum is 99.9% pure before alloying elements of silicon,chromium, magnesium, titanium, iron, and manganese are added thereto.10. An alloy composition, consisting essentially of: from about 4 wt %to about 11 wt % of silicon based on a total wt % of the alloycomposition; from about 0.1 wt % to about 0.5 wt % of chromium based onthe total wt % of the alloy composition; from about 0.1 wt % to about0.5 wt % of magnesium based on the total wt % of the alloy composition;from about 0.01 wt % to about 0.05 wt % of titanium based on the totalwt % of the alloy composition; equal to or less than 0.5 wt % of ironbased on the total wt % of the alloy composition; equal to or less than0.5 wt % of manganese based on the total wt % of the alloy composition;and a balance of aluminum.
 11. A method, comprising: reducing diesoldering during a die casting process by die casting a molten alloycomposition, including: from about 4 wt % to about 11 wt % of siliconbased on a total wt % of the alloy composition; from about 0.1 wt % toabout 0.5 wt % of chromium based on the total wt % of the alloycomposition; from about 0.1 wt % to about 0.5 wt % of magnesium based onthe total wt % of the alloy composition; from about 0.01 wt % to about0.05 wt % of titanium based on the total wt % of the alloy composition;equal to or less than 0.5 wt % of iron based on the total wt % of thealloy composition; equal to or less than 0.5 wt % of manganese based onthe total wt % of the alloy composition; and a balance of aluminum. 12.The method as defined in claim 11 wherein a total wt % of the chromiumplus the manganese plus the iron is less than 0.6 wt % based on thetotal wt % of the molten alloy composition.
 13. The method as defined inclaim 11, further comprising selecting a wt % of the chromium to reducea solubility of the iron in the molten alloy composition from about 1.5%to about 0.12%.
 14. The method as defined in claim 11, furthercomprising: die casting the molten alloy composition to form astructural body; removing the structural body from a steel die usedduring the die casting process; and wherein the structural body is notexposed to a subsequent heat-treatment process.
 15. The method asdefined in claim 14 wherein one of: i) from about 6 wt % to about 9 wt %of the silicon is present in the alloy composition; from about 0.15 wt %to about 0.2 wt % of the chromium is present in the alloy composition;from about 0.2 wt % to about 0.5 wt % of the magnesium is present in thealloy composition; from about 0.01 wt % to about 0.02 wt % of thetitanium is present in the alloy composition; equal to or less than 0.10wt % of the iron is present in the alloy composition; and equal to orless than 0.10 wt % of the manganese is present in the alloycomposition; and wherein the structural body has a ductility that rangesfrom about 10% to about 12%; or ii) from about 4 wt % to about 7 wt % ofthe silicon is present in the alloy composition; from about 0.15 wt % toabout 0.2 wt % of the chromium is present in the alloy composition; fromabout 0.2 wt % to about 0.5 wt % of the magnesium is present in thealloy composition; from about 0.01 wt % to about 0.02 wt % of thetitanium is present in the alloy composition; equal to or less than 0.10wt % of the iron is present in the alloy composition; and equal to orless than 0.15 wt % of the manganese is present in the alloy compositionand wherein the structural body has an average elongation index (EI) ofabout 7% in an as-cast condition and a yield strength of about 300 MPa.